Typically, an automatic transmission (AT) automatically shifts from one shift speed into a target shift speed on the basis of various conditions, such as speed of the vehicle, a throttle opening, etc. Such an AT often includes on-coming elements and off-going elements. These elements change during shifting, such as, while the shift speed is shifted into the target shift speed the off-going elements are disengaged by a hydraulic control and the on-coming elements are engaged by the hydraulic control. AT's typically include a torque converter, having a damper clutch, and a power-train having a shift gear mechanism which function as the on-coming or the off-going elements. However, a drawback of a vehicle having this AT is that unnecessary fuel is wasted. That is, since energy is consumed by a slip in the torque converter, the vehicle with the AT has a decreased fuel mileage in comparison with a vehicle with a manual transmission. In addition, because of such a decrease of fuel mileage, the vehicle with the AT expels an exhausting gas having an increased amount of noxious substances and such an increased level of exhaust gas brings about excessive environmental pollution.
Accordingly, to decrease such fuel waste, one previous design provided a damper clutch control method for the automatic transmission. This damper clutch control method acted to engage a damper clutch during an inertia travel under a power-off (when a driver removes their foot from the throttle) in order to intercept any slip in the torque converter. In other words, the purpose was to retain the engine RPM equal to the transmission turbine RPM. According to this method, since an engine output-shift and an AT input-shift engage each other by the damper clutch, slip in the toque converter is not generated. Therefore, since slip of the torque converter is not generated, the fuel mileage should be enhanced. Furthermore, since the damper clutch is engaged during inertia travel under the power-off situation, torque of the AT should be transmitted into an engine and the engine RPM should slowly decelerate. Accordingly, since a fuel cut time becomes longer, the fuel mileage can be enhanced.
However, the damper clutch control has drawbacks. First, since the engaged damper clutch is disengaged while the shift speed is being up-shifted into a higher speed, the engine RPM is rapidly decelerated in comparison with the turbine RPM. Accordingly, a difference between the engine RPM and the turbine RPM becomes large. Consequently, a problem occurs that the damper clutch is not engaged while the vehicle is travels in a high gear. Second, since an interchange of information does not occur between an engine control unit (ECU) and an AT control unit (TCU), a fuel-cut control and the damper clutch control are independently executed. Accordingly, efficiency is decreased. In addition, there is a range for which the damper clutch control can not be executed. For example, the damper clutch control method is not applied while the vehicle is traveling under the power-on condition or while the shift speed is being up-shifted into any speed under the power-off. Accordingly, since an application range of the damper clutch control method is very limited, efficiency of this method is decreased.
Furthermore, since the damper clutch control is executed under the power-off, oil generated by an oil pump is very small, therefore, the time for controlling the damper clutch is retarded. Moreover, since a fuel-cut control is not executed while the damper clutch control is executed, efficiency of the damper clutch method is also decreased.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.